Polarization control is required in many different types of fiber optic applications. The typical modality for obtaining this control is polarization-preserving or polarization-maintaining optical fiber. Standard single-mode optical fiber supports two orthogonal eigenmodes or polarizations. In nominally circular fibers, the polarization modes are degenerate with an identical propagation constant and group phase velocity.
Polarization-maintaining (PM) fiber is sometimes referred to as high birefringence single-mode fiber. PM fiber utilizes a stress-induced birefringence mechanism to achieve high levels of birefringence such that polarized light travels at different speeds along the orthogonal polarized axes of the fiber. Typically, these fibers embed a stress-applying region in the cladding area of the fiber. When placed symmetrically about the core, it gives a fiber cross-section two distinct axes of symmetry. These cross-sections range from elliptical to rectangular. This results in the propagation speed to be polarization dependent for light polarized along the two orthogonal symmetry axes. As a result, light propagating and polarized along one axis of symmetry does not efficiently couple into the orthogonal polarization.
One technique for quantifying the extent to which PM fiber maintains polarization is the polarization extinction ratio (PER). This ratio quantifies the degree to which light is polarized along one axis. Thus, it represents to what extent the PM fiber is maintaining polarization. Typically, there is little or no cross-coupling of optical power between the polarization modes.
In a number of different applications, controlling how light is launched into PM fiber or how light is emitted from PM fiber is useful. Semiconductor laser devices often have high polarization anisotropy. Thus, they generate light in typically one polarization depending on how the epitaxial tensile or compressive layer stresses are crafted. It is often desirable to launch this single polarization light into PM fiber and have its single polarization state maintained. Contrastingly, single mode polarized light from a PM fiber may be coupled into a polarization anisotropic device, such as a semiconductor optical amplifier (SOA). In this case, it is necessary to emit light with a polarization state aligned to the preferred polarization axis of the SOA. As a result, in these two different general applications, it can be necessary to rotationally adjust the PM fiber relative to the semiconductor optical device.
One specific example where polarization control is required is in the pump lasers used to optically pump gain fiber such as regular fiber in Raman pumping schemes or erbium-doped fiber. Polarization control is required for two different reasons.
The first is related to the fact that many times these pumps are temporally power stabilized using fiber Bragg gratings. The gratings create an external cavity that feeds-back light into the semiconductor gain medium. These semiconductor gain mediums have high polarization anisotropy. Polarization stability is thus required between the semiconductor gain medium and the fiber Bragg grating to ensure that the level of feedback seen by the pump laser is constant.
Polarization stability is also required between the pump laser chip, with or without Bragg grating stabilization, and the gain fiber in the Raman pumping scheme. Polarization isotropic fiber amplifiers are typically preferred. Raman amplification depends on whether the polarization of pump light is the same as the optical signal to be amplified, however. Thus, most Raman systems require unpolarized pump light. This is typically produced using the light from two semiconductor lasers with balanced orthogonal polarizations. PM fiber is typically deployed between the combiner and the separate pump lasers.